Calculating the pH of a Buffer Solution To prepare a buffer you weigh out 9.40 g

Question

Calculating the pH of a Buffer Solution To prepare a buffer you weigh out 9.40 grams of KNO2 and place it into a 200.00 mL volumetric flask. To this flask you add 12.0 mL of 3.00 M HNO2 and then fill it about halfway with distilled water, swirling to dissolve the contents. Finally, the flask is filled the rest of the way to the mark with distilled water. Measured volume of the weak acid Solid salt of the conjugate base Acid H2CO3 HCN HNO2 C6HsCOOH Ka 4.3 X 10-7 4.9 X 10-10o 4.6 X 10-4 6.5 X 10-5 Volumetric Flask What is the pH of the buffer that you have created?

Explanation / Answer

Solution :-

mol of KNO2 = mass/MW = 9.4/85.10379 = 0.1104

mol of HNO2 = MV = 3 * 12*10^-3 = 0.036

then...

pH = pKa + log(NO2-/HNO2)

pKa = -log(4.6*10^-4) = 3.34

pH = 3.34+ log(0.1104/0.036)

pH = 3.826

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